Conventionally, when a rotary shaft of an engine is linked to the crank shaft of the engine, a driving torque to be transmitted by the rotary shaft has a component that fluctuates, due to effects such as a combustion-expansion cycle. Consequently, the fluctuating frequency of this component rotationally vibrates gears for a fuel injection pump or gears for driving accessories such as a cooling pump, causing vibration or noise.
If a load to be driven fluctuates, as with a fuel injection pump for example, the driving torque of the rotary shaft also fluctuates, causing the driving gear attached to the rotary shaft to generate vibration or noise. Recently, the pressure of fuel injection pumps has been raised higher and higher to provide a countermeasure against exhaust emission, thereby increasing the driving torque. This, however, increases the fluctuating frequency synchronized with the fuel injection cycle, resulting in increased vibration or noise of the gears for directly driving the fuel injection pump.
For an improvement associated with the above-mentioned problem, a technology disclosed in Japanese Utility Model Laid-open No. 5-047420 was proposed. In this technology, as shown in FIG. 15, a cam shaft 52 of a fuel injection pump 51 is divided into an input section 52a and a driving section 52b. The input section 52a and the driving section 52b are coupled to each other through a damper coupling 50 composed of an inertia governor 53, a spring 54, and a damper 55. In this construction, a moment of inertia of the inertia governor 53, a spring constant of the spring 54, and a damping value of the damper 55 are appropriately selected to set a characteristic value of the damper coupling 50 to a frequency lower than a vibromotive frequency range provided by the injecting action of the fuel injection pump 51.
However, in the above-mentioned construction, coupling the input section 52a to the driving section 52b through the damper coupling 50 presents a problem of increasing the length of the cam shaft 52, thereby increasing the external size of the damper coupling in its entirety. Another problem is that, since the damper coupling 50 cannot be mounted directly on a gear of the gearing, the vibration of the gear cannot be damped. Still another problem is that, since the damper coupling 50 transmits a driving force between the input section 52a and the driving section 52b by the large power of the spring 54, the size of the spring 54 becomes large, thereby increasing the size of the damper coupling.
Further, since a vane and a rotor are pivotally inserted in a cam ring of the damper coupling 50 for rotation, the structure of the damper coupling becomes complicated and requires precision work, increasing the fabrication cost.